Barrier coatings (layers) which prevent, reduce, or inhibit the permeation of a selected substrate with a gas, vapor, chemical and/or aroma have been widely described, and such coatings are used in a variety of industries, e.g., the packaging industries, automobile industries, paint industries, tire industries etc. Typical barrier materials used in coatings include polyesters, PVDC, polyurethanes, acrylic polymers, etc.
It is well known that the barrier properties of a polymer can be improved by the addition of impermeable plate like structures. When the plates are oriented perpendicular to the diffusion (permeation) direction, the diffusing molecules must go around the plates. This leads to significant reductions in the permeability of the polymer. See, for example, E. L. Cussler et al., J. Membrane Sci. 38:161-174 (1988); W. J. Ward et al., J. Membrane Sci., 55:173-180 (1991); Chang, J. et al., Journal of Applied Polymer Science, Vol. 84, 2294 (2002); Yano, K. et al., Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997); Lan, T. et al., Chem. Mater. 6, 573 (1994); Messersmith, P. B. and Giannelis, E. P, Journal of Polymer Science A: Polymer Chemistry 33,1047 (1995); U.S. Pat. Nos. 4,528,235; 4,536,425; 4,911,218; 4,960,639; 4,983,432; 5,091,467; and 5,049,609; and International Patent Application No. WO93/04118, published Mar. 4, 1993, among others.
Control of permeation using relatively low aspect ratio platelets, at low concentrations, and thermoplastically processed at high shear rates has been previously disclosed. See, for example, E. L. Cussler et al., J. Membrane Sci. 38:161-174 (1988); L. E. Nielsen, Journal of Macromolecular Science, Chemistry A1,929, (1967); R. K. Bharadwaj, “Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites”, Macromolecules 34, 9189 (2001); G. H. Fredrickson and J. Bicerano, “Barrier properties of oriented disk composites”, Journal of Chemical Physics 110, 2181 (1999). These conditions lead to relatively small improvements in the barrier properties of the polymer. This is because the reduction in permeability varies rapidly with the aspect ratio and the concentration of plates when the plates are well aligned. If the plates are not well aligned, the reductions in permeability are further reduced. The targeted application of these earlier efforts was not coatings, but a bulk polymer with improved barrier and/or mechanical properties.
Use of platelet fillers in coating formulations is also well known. Most often, they have been used in paints to modify the rheology, enabling the production of no-drip paints. These platelet fillers are typically montmorillonites or other exfoliated silicates with aspect ratio of 50 or less. They form a house of cards type structure in the coating suspension that gives a gel like property to the paint (or coating) when it is not undergoing any shear. These structures do not have the optimally aligned plates to significantly reduce the permeability of the coating.
Use of exfoliated silicates that can produce nanocomposite barrier coatings has been achieved by several methods. The most widely used has been by combining a dissolved polymer with exfoliated filler. Water-soluble polymers such as polyvinyl alcohol (PVOH) have been combined with water exfoliated filler such as vermiculite. See, for example, Japan patent 11-246729, Sep. 14, 1999, “Gas-Barrier Poly(vinyl alcohol)/poly (acrylic acid) Compositions and their Laminates and Shaped Articles”. Sumitomo Chemical Co., Ltd. Polycarbonate dissolved in toluene has been combined with organically functionalized filler to form good barrier coatings. See, for example, W. J. Ward et al., “Gas Barrier Improvement Using Vermiculite and Mica in Polymer Films”, Journal of Membrane Science, 55:173-180 (1991). Other polymers have also been made into improved barrier coatings by dissolving them in a solvent, and using an organically functionalized filler to improve the barrier properties. See, for example, Yano, K., et al., “Synthesis and properties of polyimide-filler hybrid composites”, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997).
An alternative method that has been used to form nanocomposites has been to incorporate the exfoliated filler into the monomer before polymerization. See, for example, U.S. Pat. No. 4,472,538 “Composite Material Composed of Filler Mineral and Organic High Polymer and Method for Producing the Same”, Sep. 18, 1984; U.S. Pat. No. 4,889,885 “Composite Material Containing a Layered Silicate”, Dec. 26, 1989. In some cases, this has been done in aqueous dispersion. Several monomers that can be polymerized into elastomers had exfoliated clay incorporated into the monomer droplets before the emulsion polymerization as described in PCT Patent No. WO 97/00910, Jan. 9, 1997, “Polymer nanocomposite Formation by emulsion Synthesis”, Exxon Research and Engineering Co. Methacrylate monomer was combined with exfoliated filler in aqueous dispersion prior to its polymerization into a nanocomposite. See, for example, Lee, D. C. and Jang, L. W., Journal of Applied Polymer Science, Vol. 61, 1117-1122 (1996). None of these methods led to practical coating formulations. They were designed to help make bulk nanocomposites for thermal processing.
There are several examples of using an aqueous dispersion of exfoliated filler with an aqueous dispersion of polymer to form a nanocomposite. Most of that work used elastomeric polymers in suspension. See, for example, Wu, Y-P et al, “Structure of Carboxylated Acrylonitrile-Butadiene Rubber (CNBR)-Filler Nanocomposites by Co-coagulating Rubber Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 82, 2842-2848 (2001); Wu, Y-P et al., “Structure and Properties of Nitrile Rubber (NBR)-Filler Nanocomposites by Co-coagulating NBR Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 89, 3855-3858 (2003); Varghese and Karger-Kocsis, “Natural rubber-based nanocomposites by latex compounding with layered silicates”, Polymer (in press) (2003); Feeney et al., U.S. Pat. No. 6,087,016, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier”, Jul. 11, 2000; Feeney et al., U.S. Pat. No. 6,232,389, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier and Coated Articles”, May 15, 2001; Goldberg et al., “Nanocomposite Barrier Coatings for Elastomeric Applications”, Materials Research Society, Symposium T: Polymer nanocomposites, paper T4.7, (April 2002); and Goldberg et al., “Elastomeric Barrier Coatings for Sporting Goods”, ACS Rubber Section, Apr. 29, 2002, paper 17, published in Rubber World, vol. 226, No. 5, p 15 (August 2002). The typical use of ion exchange to make the filler surface more compatible with the polymer is not used in these references, in that usually makes the filler fall out of aqueous suspension. In order to form a nanocomposite from a combination of polymer spheres and filler platelets, one needs significant flow and deformation of the polymer. Thus it was not expected that this approach would work with more rigid, non-elastomeric polymers.
The only example found that tried this approach in a non-elastomeric polymer is described in Oriakhi and Lerner [“Poly(Pyrrole) and Poly(Thiophene)/Filler Nanocomposites via Latex-Colloid Interaction”, Materials Research Bulletin, 30, 723-729(1995)]. Their method involved forming separate aqueous polymer latex and aqueous exfoliated clay suspensions. The latex was washed repeatedly before combining with the exfoliated clay in order to remove stabilizers and surfactants. Mixing the suspensions did not lead to stable coating suspensions, but rather coagulating mixtures where the nanocomposite came out of suspension. These mixtures of clay and polymer dispersed in water could not be used as a coating formulation and are therefore very different from the invention described in this patent.
The approach used by us and described in this patent differs from the above art in that it leads to stable coating formulations that can be applied to a range of articles in order to form a nanocomposite coating. The nanocomposite forms during the drying process which is well below the melt temperature of the polymer. Thus it is clear that the polymer particles undergo significant deformation during drying.